Efforts to incorporate a diffractive optical element to an optical system have been attempted for many years, especially, a diffractive optical element, which is processed a grating height to be an equivalent to one wavelength, that is, a phase difference 2π, can concentrate lights to the 1st order diffracted light, therefore, usage aiming at achieving a small size and weight-saving and a correction of chromatic aberration of an optical system has been considered.
However, in case of a single layer diffractive optical element whose grating surface is exposed to air, the 1st order diffraction efficiency can become 100% at a reference wavelength, although other-order diffracted lights increase as the wavelength shifts away from the reference wavelength, and this becomes diffraction flare lights, there was still a problem of degrading optical performance. In order to solve this problem, a bonded-multilayer diffractive optical element having a configuration where two gratings, composed of characteristic two kinds of resins, are bonded, was devised (for example, Documents 1 and 2). That is,(n2−n1)×h=λ  (1)In the expression (1), n1 and n2 respectively denote refractive indexes (n2>n1) of the resins, h denotes a grating height of the diffractive optical element, and λdenotes a wavelength.
If equality of the expression (1) always holds within a usage wavelength range, the diffraction efficiency of the 1st order light becomes 100% within a whole range of the usage wavelength range, thereby this enables to prevent generation of diffraction flares due to the diffracted lights.